The invention relates to a control device for non-positive connections, advantageously screw connections.
Screw connections are based on the conversion of a torque into an axial force via a thread. This force results in a reversible elongation of the shaft of the screw and presses the components to be joined against one another in non-positive manner. Screw connections are numbered among the detachable connections in the construction industry, wherein their greatest danger is also found. These connections can lose their function due to vibration, settlement processes or the like.
To prevent independent detachment various methods are employed. Split pins can be fitted transversely through the screw to prevent it falling out. But then the joint has in reality already been loosened. Various types of adhesives are further used which are intended to prevent loosening due to vibration. Furthermore, elastic screws are used which maintain a certain tension and thus should prevent them vibrating loose.
All these methods, however, provide no information about the state of a screw connection and are intended to maintain a status quo which has already set in. The quality of a screw connection is determined by the axial pretensioning which in more demanding structures must be within a certain range. Thus, for constant monitoring an axial force must be continuously measured.
To measure the axial force of screw connections methods are known in which the tightening torque is converted into an axial force. These methods yield only unreliable results and are highly dependent, for example, on the coefficient of friction between the screw head and the underlying material which changes markedly with operating time.
For measuring the axial force it is, furthermore, known to apply a piezoelectric layer system on the screw head. In this way a very precise measure of the axial load in the screw connection is obtained via an ultrasonic reading device. Such a procedure, however, is associated with some disadvantages. On the one hand, it is not suitable for every shape of head and, on the other hand, a new characteristic curve must be recorded for every type of screw. Furthermore, due to the high cost of the ultrasonic read-out continuous monitoring of many screw connections is hardly economically achievable.
Known solutions have been described in DE 198 31 372 and DE 199 54 164. These solutions, however, have the disadvantage that a relatively great deformation and hence measuring distance must be used as may be seen in DE 198 31 372 with reference to FIG. 1b or 1c. A great disadvantage of the known solutions also consists in that there can be no optimized adjustment of the axial force measurement. When the piezoresistive measuring layer on a U-disk is used as presented in DE 199 54 164, the change in resistance as a function of force exhibits an exponential relationship. Consequently, with high loads the sensitivity of the measurement falls since the gradient of the curves turns out to be considerably flatter. In order nevertheless still to obtain precise results for the axial force transmitted to the screw shaft a sensitive electronic measuring system is needed or particularly costly boundary conditions have to be created so that in spite of everything a sufficiently precise and informative measurement can be carried out. This gives rise to high costs for measurements, which may still be inaccurate.
Accordingly, it is an object of the present invention to provide a device for checking non-positive connections that affords a low-cost and exact determination of the axial force introduced into the non-positive connection, in particular a screw connection.